At least 16 civilians including several children were killed in air strikes allegedly carried out by Russian jets on Thursday in Syria's Deir Ezzor province, a monitor said.

The eastern province is partly controlled by the Islamic State (IS) group, which is under pressure from several fronts in war-torn Syria.

"The civilians were killed as they tried to cross the Euphrates River near the town of Abu Kamal," the Britain-based Syrian Observatory for Human Rights said.

The Observatory - which relies on a network of sources in Syria and identifies whose planes carry out raids according to type, location, flight patterns and munitions used - said Russian jets had carried out the strikes.

Abu Kamal is one of the few remaining urban strongholds of IS in Syria, which lost control this week of Raqqa to the west, the capital of its so-called "caliphate" the militants claimed in 2014.

Russia has been criticised for its military campaign in Deir Ezzor. Military jets believed to be Russian killed at least 60 civilians trying to flee heavy fighting in the oil-rich Syrian province when their small boats were targeted as they sought to cross the Euphrates River, opposition activists, former residents and a war monitor said in early October.

They said the jets targeted makeshift rubber dinghies and boats carrying dozens of families fleeing the town of al-Ashara along the western banks of the Euphrates that lies south of Deir Ezzor city, the provincial capital.

IS's last major stronghold, the cities, towns and farms in the fertile strip along the Euphrates bordering Iraq are fast becoming the focus of Syria's six-year-long civil war.

"Russian jets staged a second wave of strikes on the boats that were fleeing across the river, causing more casualties among those who rushed to rescue earlier survivors," said Abdullah al-Akaidat, a tribal figure in northern Syria from al-Ashara who is in contact with relatives in the area.

Russia began an intervention in Syria in support of ally President Bashar al-Assad in 2015, and has helped the government win back large parts of the country.

More than 330,000 people have been killed in Syria since the conflict erupted in March 2011 with anti-government protests.